Output stability or drift over time leading to measurement inaccuracies has long been a major deficiency for gas sensors irrespective of what technology or methodology is used for their conception or realization. Output software correction may alleviate the problem somewhat but it is in many instances inaccurate and not even always applicable. Software correction has proven to be somewhat successful so far only to NDIR CO2 gas sensors used in Demand Control Ventilation application to save energy in the HVACR industry. It has long been the objective of many researchers in this field to overcome this problem fundamentally and for good. Recently the present author in U.S. application Ser. No. 12/859,749 advanced the teaching of an Absorption Biased NDIR Gas Sensing Methodology which is capable of eliminating substantially all NDIR gas sensor output drifts over time without the need for re-calibration (Wong, filed 19 Aug. 2010, now allowed to be issued 3 Jan. 2012). As it turns out, the solution to solving this output drift problem for gas sensors actually lies deeper than the availability of superior NDIR gas sensor types even though they can indeed be designed to be capable of maintaining measurement accuracy over time. The fact of the matter is that people have experienced gas sensor output instability for such a long time in the past that when output stable sensors really come along nobody will believe it. Until such time that stable gas sensors become widely available and users begin to consider their performance as trustworthy and truly believable, the real need today must be viewed at a completely different perspective and that is to be able to come up with a fast, inexpensive and simple methodology that can easily check the accuracy of gas sensors and, more importantly, just as easily and simply, hence inexpensive labor-wise, to re-calibrate them when they are found to be inaccurate.
Notwithstanding, the teaching of an Absorption Biased NDIR Gas Sensing Methodology by the present author in U.S. application Ser. No. 12/859,749 (Wong, filed 19 Aug. 2010, now allowed to be issued 3 Jan. 2012), only takes advantage of the invariance of the absorption Physics for NDIR gas measurement. As long as the changes of the sensor component characteristics over time does not affect the absorption Physics of NDIR gas measurement, the output of the sensor will remain significantly drift free. Such is the case for the aging characteristics of the infrared source whose Blackbody temperature tends to increase or decrease over time due to resistance changes causing the spectral content of its output to change. These spectral output changes can however be controlled by design by the Absorption Biased methodology not to affect the invariance of the absorption Physics for gas measurement and the outputs for such NDIR gas sensors can therefore remain significantly stable over time.
But there are situations when the changes of the sensor component characteristics alter the absorption Physics of gas measurement despite the application of the Absorption Biased methodology. When this happens, the absorption Physics invariance over time is broken and the output of the sensor will not remain stable over time. One such situation is for the filament of an incandescent infrared source (miniature light bulb) to physically sag over time. Another such situation is for a MicroElectrical Mechanical System (MEMS) infrared source to generate irregularities on its radiation emitting surface. In both cases, the ratio of the amount of radiation received by the Signal channel detector and that by the Reference channel detector is altered even when there is no gas of interest in the sample chamber thereby changing the invariance of the Absorption Physics for NDIR gas measurement. All told, it is almost impossible to design NDIR gas sensors even today whose outputs remain completely and indefinitely drift free over time. At some point in time in the future, the outputs of NDIR gas sensors will inevitably change due to the aging of sensor component characteristics affecting the invariance of the absorption Physics. It is therefore so very important to come up with a fast, inexpensive and simple methodology that can easily check the accuracy of NDIR gas sensors and more importantly, just as easily and simply, hence inexpensively labor-wise, to recalibrate them when they are found to be inaccurate.
The present author recently in U.S. application Ser. No. 13/149,738 advanced the teaching of a re-calibration methodology for Absorption Biased (AB) designed NDIR Gas Sensors (Wong, filed 31 May 2010) capable of easily and simply re-calibrating such NDIR gas sensors whose outputs have drifted over time and no longer correctly reflect their measurement accuracy. But this methodology only applies and works with Absorption Biased designed NDIR gas sensors as advanced by the present author in U.S. application Ser. No. 12/859,749 (Wong, filed 19 Aug. 2010, now allowed to be issued 3 Jan. 2012) and not with ordinary conventional NDIR nor other technology types of gas sensors. The disclosure of this re-calibration methodology for AB designed NDIR gas sensors just mentioned only dealt with the theoretical principle behind such a re-calibration technique. It is therefore the object the current invention to fill in the procedural details for carrying out such a task including the description of a special apparatus called an air sampler needed in order to perform such a re-calibration routine accurately all the time.